1. Field of the Invention
This invention relates generally to electronic devices, such as a cordless telephone. More particularly, it relates to a method and device to synchronize clocks of two wireless devices, e.g., a wireless portable device with its base device.
2. Background of Related Art
A wireless device, e.g., a cordless telephone, wireless keyboard, wireless mouse, a digital personal assistant (PDA), or a notebook computer, etc., must keep its own internal clock in synchronization with an associated base unit or station, e.g., a telephone base unit, or a desk top computer, for communications therebetween.
As a particular example, a cordless telephone system, e.g., shown in FIG. 5, generally includes a stationary unit, e.g., a base unit 10, which communicates with a telephone network (not shown), e.g., public switching telephone network (PSTN). A telephone circuit 160 of the base unit 10 typically communicates with a telephone network by wired communication means, i.e., through the tip and ring lines, 101 and 102, respectively.
The base unit 10 includes a control unit 110, which may be for example, any microprocessor, microcomputer or microcontroller known in the art and readily available from a manufacturer such as Lucent Technologies Inc. of Murray Hill, N.J. The control unit 110 controls the operation of the base unit including the communication to and from the telephone network and to and from a wireless remote handset 20. The interface unit and display 165 provides a user I/O interface to the base unit 10.
The base unit 10 is typically connected to a conventional household AC power outlet via a power circuit 190, which provides an appropriate power level to the base unit 10 derived from the household AC power. The power circuit 190 also provides appropriate power level to a charge circuit 170 that provides a charging signal to one or more charging contacts (not shown) for providing a charging power to a cradled external device, e.g., a wireless remote handset 20.
A conventional cordless telephone system, e.g., the one shown in FIG. 5, also typically includes one or more portable devices, e.g., the wireless remote handset 20, which communicates with the base unit 10 via a radio frequency (RF) link through respective RF front ends including RF circuits 150, 250 and RF transceivers 130, 230. A user of a cordless telephone system may initiate or receive a telephone call using the interface and display unit 265 of the remote handset 20 through the base unit 10, to and from the telephone network.
The remote handset 20 also includes a control unit 210, which may be for example, any microprocessor, microcomputer or microcontroller known in the art and readily available from a manufacturer such as Lucent Technologies, Inc of Murray Hill, N.J. The control unit 210 controls the operation of the base unit including the communication to and from the base unit 10. The interface unit and display 265 provides a user I/O interface to the remote handset 20.
The remote handset 20 typically includes a rechargeable battery 260. The rechargeable battery 260 provides power for the remote handset 20, and may be charged when one or more charge contacts (not shown) of the remote handset 20 are made to be in contact with one or more charge contacts (not shown) of the base unit 10, i.e., when the remote handset 20 is xe2x80x9ccradledxe2x80x9d in the base unit 10.
The frequency of the handset clock 215 must be in synchronization with the frequency of the base clock 115 in order to establish an RF communication link between the remote handset 20 and the base unit 10. Because of the differences in characteristics of the respective oscillators of the remote handset 20 and the base unit 10, the respective clocks 115 and 215 have a tendency to drift away from each other over time, and thus are required to be synchronized frequently.
The frequency synchronization of the clocks 115 and 215 may be performed at the start of each communication session between the remote handset 20 and the base unit 10. Alternatively, the synchronization may be performed periodically to maintain a reasonable alignment between the clocks in order to be able to quickly establish the RF communication link for each communication session.
Typically, in a conventional radio communication device pair, e.g., comprising the remote handset 20 and the base unit 10 of a cordless telephone system, frequency synchronization therebetween is achieved by one of the pair transmitting a radio frequency synchronization signal that is received by the other one of the pair continuously until the respective oscillators are brought into alignment using known alignment techniques. One common technique uses a Phase Locked Loop (PLL).
For example, the base unit 10 of a cordless telephone system may transmit an RF signal representing a train of pulses at its own clock rate. The remote handset 20 receives the RF signal, and xe2x80x9ctunesxe2x80x9d its oscillator by aligning its own clock signal to the received pulse rate. A more detailed description of an exemplary clock synchronization via RF signal transmission may be found in U.S. Pat. No. 5,841,823 entitled xe2x80x9cMETHOD AND APPARATUS FOR EXTRACTING A CLOCK SIGNAL FROM A RECEIVED SIGNALxe2x80x9d issued on Nov. 24, 1998 to Tuijin, the entirety of which is hereby expressly incorporated by reference. A more detailed description of a conventional oscillator alignment method, e.g., a PLL method, may be found in U.S. Pat. No. 5,463,351 entitled xe2x80x9cNESTED DIGITAL PHASE LOCK LOOPxe2x80x9d issued on Oct. 31, 1995 to Marko et al., the entirety of which is hereby expressly incorporated by reference.
Unfortunately, the above described conventional frequency (or clock) synchronization techniques require that an RF communication channel be tied up for the purpose of the synchronization, thus making the channel unavailable for user communications at least for the duration of the synchronization process.
Because the conventional synchronization is performed by transmission of signals over an RF channel, the conventional techniques are prone to error due to an RF interference. Thus, when there are many sources of RF interference, e.g. other cordless telephones in close proximity, for example, in cordless telephone manufacturing facilities or busy offices, etc., it may be impossible or at least extremely difficult to achieve clock synchronization using the above described conventional technique.
Moreover, because the operation of the RF circuit 250 consumes a significant amount of the power from the battery 270, repeated synchronization may drain the battery 270, and thus shortens the duration that the remote handset 20 may remain portable before a recharge is required.
Furthermore, under rules established by the Federal Communications Commission (xe2x80x9cFCCxe2x80x9d), a cordless telephone must employ spectrum spreading techniques (frequency hopping) if the power level exceeds a threshold level, e.g., one milliwatt, to minimize interference with other devices using the frequency band. Thus, a high power (i.e., greater than 1 mW) cordless telephone may need to continue an ongoing synchronization effort in another frequency channel thereby adding delay, consuming more power and tying up more frequency channels.
An error during the frequency hopping in a high power cordless telephone as described above may result in the handset hopping to a frequency channel different from the frequency channel of the base unit. In this case, the handset and the base unit must (during a process often referred to as the xe2x80x9cregistrationxe2x80x9d process) establish a common frequency channel in order to communicate to each other. Without a common communication channel between the handset and the base unit, the above described conventional frequency synchronization technique cannot be used for the synchronization of the clocks of the handset and the base unit, which may be a required part of the registration process.
Thus, there is a need in the art for a synchronization technique between two wireless devices, e.g., between a wireless remote handset and its base unit, which minimizes or does not require utilization of a wireless communication channel.
In accordance with the principles of the present invention, a system for and method of synchronizing a clock of a first wireless device with a clock of a second wireless device comprises means for and a step of providing a start transition and an end transition in a charge signal from the second wireless device to the first wireless device, the start transition and the end transition respectively indicating a start of a time duration and an end of the time duration during which a predetermined number of clock pulses of the clock of the second wireless device are counted, means for and a step of counting clock pulses of the clock of the first wireless device during the time duration, and means for and a step of adjusting a frequency of the clock of the first device when the counted clock pulses of the clock of the first device is different from the predetermined number of clock pulses of the second device by more than a predetermined threshold difference level.
In accordance with the principles of the present invention, a wireless device comprises a wireless base unit having a base clock, at least one remote wireless handset having a handset clock and a rechargeable power source, the remote wireless handset being adapted to receive a charge signal from the wireless base unit when cradled in contact with the wireless base unit to charge the rechargeable power source, a counter to count a number of pulses of said handset clock during a predetermined time duration between a start transition and an end transition in the charge signal; and a clock adjustment circuit adapted to adjust the handset clock when the counted number of pulses of the handset clock during the predetermined time duration differs from a number of clock pulses of the base clock counted during the predetermined time duration by more than a predetermined threshold difference level.